The present invention relates to a high pressure discharge lamp comprising a ceramic tube having a non-conductive member and a conductive member which are inserted into each end thereof, as well as a method of manufacturing such a high pressure discharge lamp.
Conventionally, such a high pressure discharge lamp has nonconductive members and conductive members tightly jointed on the nonconductive members, respectively, at both ends of a ceramic tube. When the high pressure discharge lamp is heated, for example during the operation of the lamp, a thermal stress and thus a strain by a thermal expansion occurs in a junction between the non-conductive member and the conductive member due to significant difference between the coefficient of thermal expansion of the non-conductive member and that of the conductive member. Owing to such strain, there may be formed a gap in the junction, an ionizable light-emitting material and a starting gas in a discharge space of the ceramic tube may leak from the gap to the outside of the ceramic tube.
To eliminate such a drawback, JP-A-5-290810 discloses a high pressure discharge lamp including a conductive member in the form of a support shaft and, a non-conductive member in which the support shaft is inserted. The non-conductive member is made of a plurality of layers formed of a mixture of alumina paste and tungsten paste, and arranged to cover one above the other over the surface of the support shaft in the radial direction of the ceramic discharge tube. In this case, as the layer comes closer to the central axis of the ceramic discharge tube, the volumetric percentage of the tungsten in the layer becomes higher and so does the coefficient of thermal expansion of the layer, in order to minimize the strain arising from the thermal expansion.
However, such an arrangement of the high pressure discharge lamp serves only to reduce the strain of the thermal expansion in the axial direction of the ceramic discharge tube. Strain due to thermal expansion is three dimensional and, hence, occurs also in the radial direction of the discharge tube. Therefore, when the high pressure discharge lamp is heated, an internal stress occurs at the ends of the ceramic discharge lamp. Because such an internal stress occurs repeatedly, a fatigue occurs in the ceramic discharge tube and causes cracks and chips to the ceramic discharge tube.
It is an object of the present invention to provide a high pressure discharge lamp capable of mitigating a thermal stress at an axial direction and a radial direction of a ceramic discharge tube efficiently, as well as a method of the manufacturing the same.
According to the present invention of the high pressure discharge lamp, there is provided a high pressure discharge lamp comprising:
a ceramic tube having axial ends and forming a closed inner space which is filled with an ionizable light-emitting material and a starting gas;
non-conductive members inserted into the respective ends of the ceramic tube;
a conductive member having one end which protrudes into the inner space of the ceramic tube; and
jointing means for tightly jointing the non-conductive member and the conductive member with each other, said jointing means including at least two thermal buffer layers successively stacked between the non-conductive member and the conductive member in the axial direction of the ceramic tube;
said non-conductive member, said thermal buffer layers and said conductive member having respective coefficients of thermal expansion which change gradually from the coefficient of thermal expansion of the non-conductive member to that of the conductive member.
With the above-mentioned high pressure discharge lamp according to the invention, for tightly jointing the non-conductive member and the conductive member with each other, at least two thermal buffer layers are successively stacked between the non-conductive member and the conductive member in the axial direction of the ceramic tube. The non-conductive member, thermal buffer layers and conductive member having respective coefficients of thermal expansion which change gradually from the coefficient of thermal expansion of the non-conductive member to that of the conductive member.
For example, when the non-conductive members are composed of alumina (Al2O3) and the conductive member is composed of molybdenum (Mo), a coefficient of thermal expansion of Al2O3 is higher than that of Mo. That of Al2O3 is the highest among those of Al2O3, the thermal buffer layers and Mo, and that of the thermal buffer layer directly jointed on Al2O3 is the second highest among them, the coefficient of thermal expansion gets lower as the thermal buffer layers get nearer to Mo, the coefficient of the thermal buffer layer directly jointed on Al2O3 is the second lowest among those of Al2O3, the thermal buffer layers and Mo, and that of Mo is the lowest among them.
By changing the thermal buffer layers and the conductive member gradually from the coefficient of thermal expansion of the non-conductive member to that of the conductive member, the difference of the coefficients of thermal expansion between neighboring members (between the non-conductive member and one of the thermal buffer layers, between each of the thermal buffer layers, and between one of the thermal buffer layers and the conductive members) is smaller than the case where the conductive member is directly jointed on the non-conductive member, so that strain arising from the thermal expansion at the axial direction and the radial direction of the ceramic tube is reduced. Consequently, when the high pressure discharge lamp is heated, the thermal stress at the axial direction and the radial direction of the ceramic tube can be mitigated efficiently.
Preferably, the thermal buffer layer which is directly jointed on the non-conductive member is composed of a material from which forms the non-conductive member, the thermal buffer layer which is directly jointed on the conductive member is composed of a material from which forms the conductive member.
By composing the thermal buffer layer directly jointed on the non-conductive member with a material from which forms the non-conductive member and composing the thermal buffer layer directly jointed on the conductive member with a material from which forms the conductive member in such a way, roughness of surfaces of the non-conductive member and the conductive member is buried, so that a conformability effect is obtained.
More preferably, each of the thermal buffer layers is composed of a mixture of the material from which forms the non-conductive member and the material from which forms the conductive member, the volumetric percentage of the material from which forms the conductive member in the thermal buffer layer becomes higher as the thermal buffer layer comes closer to the conductive member.
By changing the volumetric percentage in such a way, the coefficients of thermal expansion can be easily inclined, so that the thermal stress can be mitigated more efficiently.
According to the method of manufacturing a high pressure discharge lamp of the present invention, there is provided a method of manufacturing a high pressure discharge lamp, comprising the steps of:
inserting non-conductive members into respective ends of a ceramic tube which forms a closed inner space filled with an ionizable light-emitting material and a starting gas;
successively stacking at least two thermal buffer layers on an outer face of the non-conductive member in the axial direction of the ceramic tube so as to be tightly jointed on the non-conductive member; and
jointing a conductive member to the thermal buffer layers such that one end of the conductive member protrudes into the inner space of the ceramic tube to thereby form a structure wherein coefficients of the thermal expansion of the non-conductive member, the thermal buffer layers and the conductive member change gradually from the coefficient of thermal expansion of the non-conductive member to that of the conductive member.
With the above-mentioned method according to the present invention, first non-conductive members are inserted into respective ends of a ceramic tube which forms a closed inner space filled with an ionizable light-emitting material and a starting gas. Secondly, at least two thermal buffer layers successively stack on an outer face of the non-conductive member in the axial direction of the ceramic tube so as to tightly joint on the non-conductive member. Lastly, a conductive member is jointed to the thermal buffer layers such that one end of the conductive member protrudes into the inner space of the ceramic tube to thereby form a structure wherein coefficients of the thermal expansion of the non-conductive member, the thermal buffer layers and the conductive member change gradually from the coefficient of thermal expansion of the non-conductive member to that of the conductive member.
By manufacturing the high pressure discharge lamp in such a way, it is possible to manufacture a high pressure discharge lamp capable of mitigating a thermal stress at an axial direction and a radial direction of a ceramic discharge tube efficiently.
Preferably, the thermal buffer layer which is directly jointed on the non-conductive member is composed of a material from which forms the non-conductive member and, the thermal buffer layer which is directly jointed on the conductive member is composed of a material from which forms the conductive member.
In this case, it is possible to manufacture the high pressure discharge lamp capable of obtaining the conformability effect.
More preferably, each of the thermal buffer layers is composed of a mixture of the material from which forms the non-conductive member and the material from which forms the conductive member. In such a configuration, volumetric percentage of the material from which forms the conductive member in the thermal buffer layers becomes higher as the thermal buffer layer comes closer to the conductive member.
In this case, it is possible to manufacture the high pressure discharge lamp capable of inclining the coefficients of thermal expansion easily and thus mitigating the thermal stress at the axial direction and the radial direction of a ceramic discharge tube more efficiently.
More preferably, each of the thermal buffer layers is formed by printing a paste or a mixture of pastes.
As the thermal buffer layers are formed with one or more flexible soft pastes, the conformability to the non-conductive member and the conductive member is improved, so that the thermal buffer layers can be easily formed.